There are two types of plant defenses: constitutive and inducible. Constitutive defenses such as cell walls, waxy cuticles, and bark are always present in plants, while inducible defenses like toxic chemicals and programmed cell death are produced when the plant is injured or detects pathogens. Secondary compounds found in many plants are also defensive as they are toxic to herbivores and microbes, and plants increase production of these compounds when under attack. Examples of medically useful secondary compounds include menthol, carotene, and sennoside.